This is a highly debated subject and pretty much depends on the soil, region, and plants. Pine needles are naturally acidic in nature. As they decompose, which is a slow process, additional acid is added to the soil. However, this may not necessarily be a dramatic increase. Since pine needles are acidic, it goes without saying that acid-loving plants (like blueberries, etc.) thrive with this type of mulch. While spruce is also acidic, though perhaps not as much, you could in fact use this as a substitution for pine. However, I wouldn't rely so heavily on this alone to acidfy the soil. I would first have the soil tested, which will let you know how acidic the soil may already be (or not), and then go from there. This article has information on other ways to raise the acidity in soil: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/raise-acid-level-soil.htm This article may help as well: https://www.gardeningknowhow.com/edible/fruits/blueberries/lower-soil-ph-blueberries.htm